1. Field of the Invention
The invention relates to installing an optical fibre cable in a duct and more particularly to installing an optical fibre cable in a duct by propelling the cable along the duct by means of a gas flow.
2. Description of the Related Art
The installation of optical fibre cables using a gas to propel the cable along a duct, which can be referred to as ‘blown cable installation’, is known. Blown installation of optical fibres is, for example, disclosed in EP-A-0108590 and EP-A-0770894.
The introduction of the blown cable installation process has led to the design of specialised small lightweight equipment, which is used to propel an optical fibre cable along a duct. The main component of such equipment is referred to as the blowing head, which comprises a cable guide, a hydraulically driven caterpillar capstan and an air box. The air box is where the optical fibre cable and compressed air, at approximately 13 Bar, are introduced into a duct along which the cable is to be fed. The caterpillar capstan is positioned upstream of the air box and is arranged to pull the optical fibre cable from a cable drum and assist its feed into the duct. The capstan typically consists of opposed belts that comprise rubber blocks mounted on a chain and are driven at speeds of up to 60 m/min.
An important parameter for determining the distance an optical fibre line can be installed by the blown cable installation process is the amount of push force, or assist, provided by the caterpillar capstan. Where the compressed air and fibre line are introduced into the duct, there is a dead zone at which the air flow is insufficient to propel the cable along the duct. The caterpillar capstan pushes the optical fibre line through the dead zone to enable installation to take place.
When planning an installation, the maximum blowing distance must be taken into account. Should this distance prove unachievable in practice, considerable extra work is required to complete the installation. If the installer takes a cautious approach and does not attempt installation over the maximum blowing distance, the number of installation procedures is increased thereby increasing the overall cost of the installation.
One reason the maximum blowing distance may not be achieved is that the level of the assist provided by the caterpillar capstan may be reduced if the outer surface of the optical fibre cable is wet. If the cable is wet, the moisture it carries is wiped off by the belts of the caterpillar capstan, which eventually become saturated causing them to slip over the cable. The resultant loss of friction between the optical fibre cable and the belts leads to a reduction in the assist provided by the caterpillar capstan and can result in the maximum blowing distance not being achieved. This problem is particularly likely to occur if the installation takes place in the open during wet weather conditions.
An object of the invention is to at least partially overcome this problem.